Event detection device with fault monitoring capability

ABSTRACT

An event detection device comprising a sensing means for generating an output signal in response to the detection of an event, the device comprising a fault monitoring system which comprises: comparator means for comparing the output signal with a threshold signal and for activating a timed sequence processor when the output signal exceeds the threshold signal on a first occasion, the time sequence processor being adapted to activate a fault indicating circuit after a predetermined time interval unless the sensing means generates an output signal in response to the detection of an event on a second occasion within the predetermined time interval.

FIELD OF THE INVENTION

This invention relates to event detection devices and more particularlyto an event detection device having an anti-masking capability.

BACKGROUND ART

Event detection devices, for example, intrusion monitoring devices, arewell known in the art. Typically they are used to detect unauthorizedentry or intrusion into a protected space.

Commercially available intrusion monitoring devices can be either of thepassive or active variety. Passive intrusion monitoring devices can, forexample, comprise a sensor which detects infra red radiation propagatedby warm blooded animals. Typically such passive devices comprise athermal detection device, consisting of one of more thermal detectorsadapted to detect infra red radiation incident thereon, and an opticalsystem for directing incident radiation from a plurality of angularfields of view towards the thermal detection device. Such opticalsystems may consist of lenses, particular Fresnel lenses and/orreflecting surfaces. Normally such devices are activated when a sourceof infra red radiation passes from one angular sector to the next.Typical prior art intrusion monitoring devices are illustrated in U.S.Pat. Nos. 3,703,718 and 3,958,118, and in UK patent number 1335410. Theentire disclosures of all these patents are included herein by referencefor all purposes.

Active intrusion monitoring devices are also known which comprise atransmitter and a receiver, the transmitter emitting radiation at adefined frequency and a receiver measuring the Doppler shift in anyreflective radiation. Such active devices can, for example, operate atmicrowave frequencies, using a microwave radiation detection device todetect the reflective radiation.

Whatever detection device is used, it is necessary in each case toprovide an electrical circuit to process the electrical output signal ofthe detection device and to compare that signal with a pre-set thresholdsignal.

The above devices can be used alone, or as a combined technology eventdetection device. Examples of such combined devices includingspecifically a combination of a photo electric sensor and a microwavesensor are shown in U.S. Pat. Nos. 3,725,888 and 4,401,976, the entiredisclosures of which are incorporated herein by reference for allpurposes.

In a typical combined technology event detection device, the outputs oftwo independent sensing means, responding to different physical stimuli,are supplied to an AND gate, and if both sensing means register an eventwithin a specified period of time, then an alarm is triggered. In thismanner the incidence of false alarms occurring when only a single sensormeans is used can be greatly reduced.

A problem with both single and combined technology event detectiondevices is that if the detector is masked, for example, by placing ascreen in front of the detector which will absorb the microwave signalsemitted by the microwave device, or which will block infra red signalsand prevent them from reaching the passive infra red sensor, the eventdetection device is rendered inoperable.

Attempts have been made to overcome this problem by providing the eventdetection device with a separate system comprising an infra red LEDemitter and a detector which operate at a frequency range different fromthat of the passive infra red sensor. If an object is placed near theevent detection device so as to mask the passive infra red sensor, theinfra red LED/detector system will detect the presence of the object andcause an alarm to be triggered.

Such anti-masking system increase the expense of the device, and in somecircumstances are ineffective, because it is still possible to mask allor part of the Fresnel lens associated with the passive infra red sensorwithout traversing the light beam from the infra red LED. Thus a skilfulthief can mask the lens without activating the anti-masking system.

U.S. Pat. No. 4,833,450 discloses an event detection in which the alarmis sounded if a signal from a masking circuit exceeds a threshold level.The alarm continues to sound for a predetermined period. Once thepredetermined period has lapsed and the correct of operation of theevent detection device has been confirmed, the alarm is reset.

SUMMARY OF THE INVENTION

The present invention provides an event detection device provided with afault monitoring system, such that, when the event detection device isconnected to an alarm system, and the alarm system is armed, the eventdetection device will indicate a fault condition if the device has beentampered with or is defective, or has been accidentally or deliberatelymasked.

The invention provides an improved fault monitoring system for an eventdetection device wherein the fault monitoring and/or anti-masking systemis activated by a signal generated by the event detection device.

According to the present invention there is provided an event detectiondevice comprising a sensing means for generating an output signal inresponse to the detection of an event, and a fault monitoring systemresponsive to the output signal for indicating the presence of a fault,masking, or tampering with the device, wherein the fault monitoringsystem is responsive to an output signal from the sensing meansindicating the detection of an event proximate to the event detectiondevice.

In a first aspect, the invention provides an event detection devicecomprising a sensing means for generating an output signal in responseto the detection of an event, the device comprising a fault monitoringsystem which comprises:

comparator means for comparing the output signal with a threshold signaland for activating a timed sequence processor when the output signalexceeds the threshold signal on a first occasion,

the timed sequence processor being adapted to activate a faultindicating circuit after a predetermined time interval unless thesensing means generates an output signal in response to the detection ofan event on a second occasion within the predetermined time interval.

The invention is applicable to both single technology and combinedtechnology event detection devices.

In a second aspect, the invention provides a combined technology eventdetection device which comprises a first sensing means for generating afirst output signal in response to the detection of an event, a secondsensing means for generating a second output signal in response to thedetection of an event, logic means for receiving the first and secondoutput signals and for generating an alarm in response thereto, and afault monitoring system comprising:

comparator means for comparing the first output signal with a thresholdsignal and for activating a timed sequence processor when the outputsignal exceeds the threshold signal on a first occasion,

the timed sequence processor being adapted to activate a faultindicating circuit after a predetermined time interval unless the firstsensing means generates a first output signal in response to thedetection of an event, and the second sensing means generates a secondoutput signal in response to the detection of an event, within thepredetermined time interval.

The event detection device may be of the type used, for example, todetect movement and/or body temperature, and may be, for example, anintrusion detection device. Other uses of the device are, however, alsopossible. The invention is applicable to single technology eventdetection devices, for example, passive infra red sensor devices, and tocombined technology event detection devices, for example, thosecomprising a passive infra red sensor and a Doppler shift microwavesensor, for example, of the type sold by Pyronix Limited under the trademark EQUINOX. More than two sensing means may be used where necessary ordesired, in which the case the logic means may generate an alarm inresponse to the summation of the output signals received from two ormore of the sensing means.

The logic means may be included within one or more micro processorswhich can interrogate the sensing means for activity. The logic meanscan comprise any suitable logical algorithm, for example, a logic ‘AND’function or a pulse counting function.

Preferably the output signal from the sensing means is fed into atwo-stage amplifier and the comparator means is connected to the outputfrom the first stage of the two-stage amplifier. In such an arrangement,the output from the second stage of the two-stage amplifier may beconnected to a second comparator means for comparing the amplifiedoutput signal with a second threshold signal. The second comparatormeans may have an output state and be adapted to change said outputstate when the amplified electrical output signal traverses the secondthreshold signal, the changed output state indicating an alarmcondition.

Preferably the timed sequence processor has a first passive state for aperiod wherein the sensing means is allowed to return to a passive orinactive condition, and a second active state for a period wherein thetimed sequence processor interrogates the sensing means in order todetect an output signal from the sensing means in response to thedetection of an event. If an output signal indicating a distant event isreceived the timed sequence processor is re-set and that sequenceterminated. If an output signal indicating a proximate event isreceived, the timed sequence processor is re-set and re-started. If nooutput signal is detected, the timed sequence processor activates afault indicating circuit which remains in an activated state untilswitched off. In a preferred embodiment according to the invention, thefault indicating circuit is rendered inactive by the generation of anoutput signal from the sensing means in response to the detection of afurther distant event.

Preferably, if the fault indicating circuit is activated, the outputstate of the second comparator, or other alarm circuit, is also changedto indicate an alarm condition.

The predetermined time interval can be of any convenient length, and,for example, time periods within the range of from 5 seconds to 5minutes have been found to be suitable. Preferably the time interval isaround 30 seconds, with a passive state of 15 seconds and a furtheractive state of 15 seconds.

The comparator means and timed sequence processor can be included in oneor more micro processors as appropriate.

The fault indicating circuit can comprise a visual indication means, forexample an LED, or may simply provide an electrical signal, for example,it can comprise a switch which remains open circuit whilst activated.

The event detection device of the invention can also comprise a faultmonitoring system in accordance with International Patent applicationNo. WO95/28692, the entire disclosure of which is incorporated herein byreference for all purposes. The outputs of the two fault monitoringsystems can be combined or separate.

DESCRIPTION OF DRAWING

Embodiments of event detection devices according to the invention willnow be more particularly described, by way of example only, withreference to the accompanying Drawings in which:

FIG. 1 shows a schematic block diagram of a single technology eventdetection device according to the invention;

FIG. 2 shows a schematic block diagram of a combined technology eventdetection device according to the invention;

FIGS. 3(a), (b), (c) and (d) shows the signals at points X and Y in FIG.1 when an event is detected at 10 meters and at 50 cm;

FIG. 4 shows the signals at points X and Y in FIG. 2 when an event isdetected at a distance of 10 meters and at 50 cm; and

FIG. 5 shows a graph of the time sequence of the timed sequenceprocessor indicating the possible steps following the detection of anevent.

Referring firstly to FIG. 1, there is shown a passive infra red sensor 1having an output connected to a two-stage amplifier 2. The output of thefirst stage 3 of the two-stage amplifier 2 is connected to a firstcomparator 4 which in turn is connected to a timed sequence processor 5.

The output of the second stage 6 of the two-stage amplifier is connectedat point Y to a second comparator 7. The output of the second comparator7 is connected to the timed sequence processor 5, and to an alarm output8.

The output of the timed sequence processor 5 is connected to a faultoutput 9 and to the alarm output 8.

The signals at points X and Y in FIG. 1, corresponding to the detectionof an event, are illustrated in FIG. 3. FIG. 3a shows the signal atpoint X when an event is detected by the passive infra red sensor 1 at adistance of 10 meters or more. The signal, though amplified by amplifierstage 3, is still extremely small. The same signal, after passingthrough amplifier stage 6 is shown in FIG. 3b. It can be seen that thesignal exceeds the threshold t₁ and is sufficient to give a signal fromthe second comparator 7 to alarm output 8. If, however, the system towhich the event detection device is connected is not armed, no alarm issounded, and the device will return to its inactive state after apre-set interval, usually about 1 second.

The effect of an event being detected at 50 cm distance, or less, isshown in FIGS. 3c and 3 d. From FIG. 3c it can be seen that the signalat point X, the output of the first amplifier stage 3, is quite large,but irregular. The signal at point Y, the output of the second amplifierstage, has overloaded the system. This larger signal will, of course,also activate the alarm output via the comparator 7. However, becausethe signal at point X, is also greater than the threshold t₂ of thefirst comparator 4, the timed sequence processor 5 will also beactivated.

The timed sequence processor 5 has a passive state lasting for about 15seconds to allow the passive infra red sensor to return to its inactivecondition. There follows a further 15 seconds when the timed sequenceprocessor waits for a signal from comparator 7 to confirm that theprocessor 5 can be re-set. If a signal indicating a distant event (10 m)is received, the processor is re-set and the sequence terminated. If asignal indicating a proximate event (50 cm) is received, the processoris re-set and the sequence re-started. If no such signal is received,either because there is a fault in the system, or because the passiveinfra red sensor 1 has been masked, the timed sequence processor 5 sendsan output signal indicating a fault condition to a fault indicatingoutput 9.

Unlike the alarm output, which last only for about 1 second, the circuit9 indicating the fault remains active, such that when the alarm systemto which the event detection device is connector is armed, the faultcondition continues to be indicated, and will inform the alarm systemuntil the fault is corrected. Similarly, because the timed sequenceprocessor is also connected to the alarm output 8, the alarm circuitwill also remain activated.

It can be seen that, in this way, the passive infra red sensor cannot bedisabled by masking whilst the alarm system is un-armed, without thisfact becoming apparent to an operator seeking to arm the system.

Referring now to FIG. 2, there is shown a combined technology eventdetection device comprising a microwave sensor 10 and a passive infrared sensor 11. The output of the microwave sensor 10 is connected to theinput of a first two-stage amplifier 12, having a first stage 13 and asecond stage 14.

The output from the first stage 13 is connected at point X′ to a firstcomparator 15, which is connected to a timed sequence processor 16,which in turn is connected to a fault output 17.

The output from the second stage 14 of the two-stage amplifier 12 is fedat point Y′ into a second comparator 18, and from thence to a logic‘AND’ function 19 which is connected to the timed sequence processor 16and to an alarm output 20. The output of the timed sequence processor 16is also fed to the alarm output 20.

The output from the passive infra red sensor 11 is connected to theinput of a second two-stage amplifier 21, the output of which is fed toa third comparator 22. The third comparator 22 is connected to the logic‘AND’ function 19.

The signals at points X′ and Y′ due to the detection of an event areshown in FIG. 4. When an event is detected at a distance of 10 meters bythe microwave sensor 10, a very small signal is observed at point X′ asshown in FIG. 4(a). This signal is amplified by the second stage 14 ofthe amplifier 12, and appears as shown in FIG. 4(b) at point Y′.

The effect of an event detected by microwave sensor 10 at a distance of50 cm or less is shown in FIG. 4(c) and FIG. 4(d). In FIG. 4(c), it canbe seen that there is a substantial signal at point X′ which hasexceeded the threshold t₃. The signal at point Y′, shown in FIG. 4(d),is sufficient to produce an overload condition.

Because the signal at point X′ has exceeded the threshold t₃ of thecomparator 15, the timed sequence processor 16 is activated. A graph ofthe timed sequence in shown in FIG. 5.

For the first 15 seconds the sequence processor 16 remains in a passivestate, waiting for the microwave sensor 10 to return to its inactivecondition. For the next 15 seconds the timed sequence processor 16interrogates both the microwave sensor 10 and the passive infra redsensor 11, through the logic ‘AND’ function output, seeking confirmationof the event. If both sensors indicate that a distant event has occurredwithin the second 15 second period, the timed sequence processor isre-set and returns to its waiting condition. This is indicated by 2ticks in the block diagram in FIG. 5.

If, in the second 15 second period, only one of the sensors 10, 11indicates that an event has occurred (one tick and a cross), or neithersensor indicates that an event has occurred (two crosses), at the end ofthe period the timed sequence processor 16 will send a signal to a faultoutput circuit 17. The fault output circuit 17 remains open until thefault has been corrected. If the microwave sensor indicates that aproximate event has occurred, the timed sequence processor is re-set andre-started.

Because the time sequence processor 16 is connected to both the faultoutput 17 and the alarm output 20, both will remain activated until theprocessor is re-set.

It should be stressed that the fault monitoring system is activated onlywhen a signal is received indicating that an event has been detectedwithin a short distance from the sensor, and the timed sequenceprocessor would normally be re-set by the detection of a further distantevent within its second 15 sec period of operation. Only if the timedsequence processor does not receive confirmation of an event within itssecond 15 second period will the fault output circuit be activated.

Throughout the operation of the fault monitoring system the device willcontinue to operate as a combined technology event detection device, andif both sensors 10, 11 indicate that an event has occurredsimultaneously the alarm output 20 will be activated via the logic ‘AND’function 19.

Whilst the fault monitoring system of the present invention may beuseful in detection electrical faults in, or tampering with, the eventdetection device, its most important application is as an anti-maskingsystem in the prevention of accidental or deliberate masking of theevent detection device, which, for the purposes of this specification,is also described herein as a fault condition.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

What is claimed is:
 1. An event detection device, comprising: a firstsensor to generate a first output signal in response to the detection ofan event, the first sensor generating a second output signal to preventactivating a fault indicating circuit upon detecting the event on asecond occasion within a predetermined time interval; and a comparatorto compare the first output signal with a threshold signal and activatea timed sequence processor when the first output signal exceeds thethreshold signal on a first occasion, the timed sequence processor beingadapted to activate the fault indicating circuit after the predeterminedtime interval unless the first sensor generates the second output signalin response to the detection of an event on the second occasion withinthe predetermined time interval.
 2. An event detection device accordingto claim 1 wherein the device further comprises a second sensor togenerate a second output signal in response to the detection of anevent, logic to receive the first and second output signals and togenerate an alarm in response thereto, and a fault monitoring systemhaving a comparator to compare the first output signal with a thresholdsignal and to activate the timed sequence processor when the firstoutput signal exceeds the threshold signal on a first occasion, thetimed sequence processor being adapted to activate a fault indicatingcircuit after a predetermined time interval unless the first sensorgenerates another first output signal in response to the detection of anevent, and the second sensor generates another second output signal inresponse to the detection of an event, within the predetermined timeinterval.
 3. An event detection device according to claim 1 to detect anintrusion.
 4. An event detection device according to claim 1, whereinthe first sensor is a passive infra red sensor.
 5. An event detectiondevice according to claim 1, wherein the first sensor is a Doppler shiftmicrowave sensor.
 6. An event detection device according to claim 1,further comprising a two stage amplifier in which the output signal fromone amplifier and the comparator is connected to the output from thefirst stage of the two-stage amplifier.
 7. An event detection deviceaccording to claim 1, in which the timed sequence processor has a firstpassive state for a period wherein the first sensor is allowed to returnto a passive or inactive condition after the detection of an event, anda second active state for a period wherein the timed sequence processorinterrogates the first sensor in order to detect an output signal fromthe first sensor in response to the detection of an event.
 8. An eventdetection device according to claim 1, in which the arrangement is suchthat, when the timed sequence processor is activated, if an outputsignal indicating a distant event is received the timed sequenceprocessor is re-set and that sequence terminated, and if an outputsignal indicating a proximate event is received the timed sequenceprocessor is re-set and re-started, and if no output signal is detectedthe timed sequence processor activates a fault indicating circuit.
 9. Anevent detection according to claim 1, in which the fault indicatingcircuit is rendered inactive by the generation of an output renderedinactive by the generation of an output signal from the first sensor inresponse to the detection of a distant event.
 10. An event detectiondevice according to claim 1, in which, if the fault indicating circuitis activated, the output state of a second comparator, or other alarmcircuit, is also changed to indicate an alarm condition.
 11. An eventdetection device according to claim 1, in which the predetermined timeinterval is from 5 seconds to 5 minutes.
 12. An event detector accordingto claim 1, further comprising one or more additional sensors coupled tothe timed sequence processor to generate one or more confirmatory outputsignals in response to the event.
 13. An event detection device,comprising: a sensor to generate an output signal in response to thedetection of an event, the sensor generating a second output signal toprevent activating a fault indicating circuit upon detecting the eventon a second occasion within a predetermined time interval; and a timedsequence processor coupled to the sensor and adapted to be triggered ona first occasion to activate a fault indicating circuit after thepredetermined time interval unless the sensor generates the secondoutput signal in response to the detection of an event on a secondoccasion within the predetermined time interval; and one or moreadditional sensors coupled to the timed sequence processor to generateone or more confirmatory output signals in response to the event.
 14. Anevent detection device according to claim 13, further comprising acombining device positioned between the timed sequence processor and thesensors to supply output signals from the sensors to the timed sequenceprocessor in response to the event.
 15. An event detection deviceaccording to claim 14, wherein the combining device is an and device.